Army ranks: Specialist and Corporal?

In addition to Wraeth's great answer, some background on the two ranks.

Once upon a time, Specialist-4 through Specialist-7 were designed to provide those SOldiers in a technical MOS, who were not in the leadership track, an opportunity to exercise some administrative leadership, job growth, and promotion opportunities. Additionally, at the time the Army was limited by law as to how many Soldiers could be in a specific NCO grade. The Specialist system was also a legal way around any limitations.

As time went on, the Army made the decision to do away with the SPC-5 through SPC-7 grades for a variety of reasons - most of which were crap. The main reason was that the thought process was that if you were going to be a SPC-7 in charge of an office, then you needed to be an NCO in charge of that office. There were also a lot of instances when a long-service, experienced SPC-6 or -7 would be subordinate to some young Sergeant on a detail, simply because the Sergeant was an NCO.

After SPC-5, -6, and -7 were done away with, the SPC-4 (to eventually become SP4, and then just SPC) was retained, amongst other reasons, to allow for growth as a junior leader and to experience in one's MOS without having to worry about NCO duties.

Corporal is a junior NCO rank, but there are very few positions on any TOE that are coded for an actual Corporal - we simply do not break our echelons down that far unlike most other western Armies. Being laterally transferred to Corporal from SPC eventually became a system of reward for promising young Soldiers and often lacked the emphasis on actual junior leader training. There was a urban myth in the Army for a while that only Combat Arms Soldiers could be made a Corporal, but that is patently false. While possible, I've never see anyone actually promoted directly to Corporal from PFC. In my experience they were always laterally transferred after showing promise.

A Corporal and a Specialist is paid the same based on years service. There is a difference in status, as one is technically an NCO, and one is not.

Everyone is promoted to Specialist. Very very few Corporal promotions are ever given anymote.. it is only to be given to Specialists who are holding an E-5 position. Most Commanders though will jot sign off on a Corporal promotion because they say if they are good enough to be an NCO, just promote them to SGT.